The present invention relates to the testing of buried electrical cables in order to detect faults in the cable.
For esthetic and other reasons, electric utility cables are often buried underground running thousands of feet between terminal points. High voltage cables typically have two conductors, an insulated phase conductor and a bare neutral conductor consisting of a number of wires that are separately helically wound about the length of the first conductor. The neutral conductor also acts as a shield for the phase conductor against electrical stress. This type of cable is commonly referred to as a concentric neutral cable. Three such cables are employed for three phase electrical service.
Although this type of concentric neutral cable which employs a bare neutral conductor is relatively inexpensive compared to providing an insulated neutral conductor, the bare conductor is prone to corrosion. After prolonged exposure to certain types of soil conditions, the wires of the bare neutral conductor begin to corrode eventually producing an open circuit. Not only does the breakage of even a few neutral wires affect the current carrying capacity of the neutral conductor, but the shielding of the phase conductor is also degraded.
Because the cable is buried, visual observation cannot be employed to detect the location of a break or to forwarn of the onset of significant corrosion. Although conventional resistivity measurements can be employed to detect the section of cable in which breaks in the neutral conductor wires occur, such techniques cannot pinpoint the precise location of the breaks. It is relatively expensive and time consuming to dig up long sections of the cable in order to pinpoint the location of significant corrosion in the concentric neutral conductor.
Equipment is available for detecting breaks in insulated cable which is buried underground. Such equipment couples an alternating signal at a given frequency between an insulated conductor of the cable and earth ground at one end of the cable. The other end of the insulated conductor is connected directly to earth ground. The signal flows into the earth at the point of a fault in the conductor insulation creating a voltage potential variation between probes inserted in the earth at different locations above the cable's path. This potential change is detected in order to locate the break. This technique cannot be applied to detect faults in bare neutral conductors as the signal will flow into the ground all along the conductor.